The present invention relates to rubber vibration insulators having an ultra-low hardness which are used for vibration insulation of precision instruments such as acoustic, computer-associated and game instruments.
Recently, precision instruments such as acoustic, computer-associated and game instruments are often loaded with a laser disk device. The laser disk device is very sensitive to vibration, and therefore, the device is equipped with a material for vibration insulation, vibration damping or shock absorption.
In general, it is desirable for such a vibration insulator to have a low hardness, a small compression set (c-set), a large loss tangent (tan-xcex4) and a small temperature dependence (for example, indicated by a variation of Young""s modulus in the range of 0xc2x0 C. to 80xc2x0 C.). Also, a raw composition used for preparing such a vibration insulator should be easily prepared (in particular, it must have a good kneading property), and should have excellent processability.
Such a vibration insulator is usually prepared from a thermoplastic or thermosetting composition. Vibration insulators prepared from a thermoplastic composition are described, for example, in JP-A-235477/1997 and JP-A-263702/1997, but the insulators have drawbacks such as a large compression set and a large temperature dependence.
On the other hand, a vibration insulator containing a butyl rubber as a major component is known as that prepared from a thermosetting composition (JP-A-3278/1997 and JP-A-71700/1997). However, the insulator has a limit in obtaining a low hardness. Thus, in order to obtain a vibration insulator having a low hardness, a large amount of an extender component such as a process oil should be added to the raw composition for the insulator. However, this measure deteriorates the kneading property of the raw composition (the raw composition adheres to rolls), and causes the bleeding of the process oil. Even if a vibration insulator having a low hardness is prepared from a raw composition using a special method, the resulting insulator has a large compression set.
Also, a vibration insulator containing polynorbornene as a major component is known as that prepared from a thermosetting composition (JP-U-72795/1988 and JP-Y-52435/1995). However, the insulator has an inferior thermal property. In addition, the kneading property of the raw composition and the bleeding property of the insulator are not satisfactory.
Furthermore, JP-B-43865/1985 discloses a vibration damper which is obtained by press molding a composition comprising a silicone rubber, a partially crosslinked butyl rubber, an extender oil and a reinforcing filler. However, the vibration dampers disclosed in this document have a hardness of 18 to 25.5 measured by a spring-type hardness tester, and therefore, the dampers do not aim at an insulator having a durometer hardness (JIS K6253) of not greater than A15 as defined by the present invention.
Moreover, JP-A-324167/1995 discloses a vibration damper which is obtained by previously mixing a low-molecular material such as a process oil and a medium material such as a thermoplastic organic polymer to prepare a first mixture containing the low-molecular material and the medium material, then mixing the first mixture with a polymer such as natural rubber to prepare a second mixture, and then vulcanizing the second mixture in the presence of a curing agent such as sulfur. Although a low hardness of 15 measured by Asker-C type hardness tester (25xc2x0 C.) is realized in this vibration damper, it does not have a satisfactory compression set property.
As described above, it is desirable for a vibration insulator used in precision instruments such as acoustic, computer-associated and game instruments to have a low hardness, a small compression set, a large loss tangent and a small temperature dependence. Also, a raw composition used for preparing a vibration insulator should be easily prepared and have an excellent processability. However, conventional vibration insulators do not meet all the above requirements. Accordingly, the present inventors intended to provide a vibration insulator which meets all the above requirements.
Intensive investigation has been made in order to solve the above problem. As a result, it has been found that a partially crosslinked butyl rubber shows a good kneading property even if a large amount of an oil is added to the rubber. Also, it has been found that a partially crosslinked butyl rubber shows a high retention of the oil and can inhibit its bleeding. Furthermore, it has been found that, on crosslinking and curing such a partially crosslinked butyl rubber containing a large amount of an oil, a vibration insulator having a very low hardness, a small compression set, a large loss tangent and a small temperature dependence is obtained.
Thus, the present invention provides a rubber composition comprising 100 parts by weight of a partially crosslinked butyl rubber and 50 to 200 parts by weight of an extender component.
The rubber composition according to the present invention may further contain fillers, vulcanizing agents, processability-improving polymers and/or other additives for rubber.
Also, the present invention provides a rubber vibration insulator which is obtainable by crosslinking and curing the above rubber composition and which has a durometer hardness (JIS K6253) of not greater than A15 and preferably of not greater than A10, an Asker-C hardness of at least 10 and preferably of at least 15, a compression set (JIS K6262; 70xc2x0 C.xc3x9724 hrs) of not greater than 20% and a loss tangent (25xc2x0 C., 30 Hz) of at least 0.1.
A partially crosslinked butyl rubber used in the present invention is, for example, one obtained by adding a vinyl aromatic compound (styrene, divinylbenzene, etc.) and an organic peroxide to a butyl rubber and partially crosslinking the butyl rubber, as described in JP-A-107738/1994, or one obtained by adding an electron withdrawing group-containing polyfunctional monomer (ethylene dimethacrylate, trimethylolpropane triacrylate, N,Nxe2x80x2-m-phenylene dimaleimide, etc.) and an organic peroxide to a butyl rubber and partially crosslinking the butyl rubber, as described in JP-A-172547/1994.
The degree of crosslinking of these partially crosslinked butyl rubbers may be expressed as the amount of insoluble residues (gel content) not dissolved in a solvent such as diisobutylene or cyclohexane which can completely dissolve a butyl rubber in an unvulcanized state. The partially crosslinked butyl rubbers used in the present invention have a gel content in the range of 5% to 95% when using the above solvent. Among others, butyl rubbers having a higher gel content are more preferable because they can retain a larger amount of an extender component. Accordingly, the gel content of the partially crosslinked butyl rubbers is preferably in the range of 50% to 95%, and more preferably in the range of 65% to 95%.
One preferable example of partially crosslinked butyl rubber is a terpolymer comprised of isobutylene, isoprene and divinylbenzene which is partially crosslinked with divinylbenzene. Such a partially crosslinked butyl rubber is commercially available as Polysar Butyls XL 10000, XL 68102, XL 30102 and XL 40302 (Polysar International Co.). Among them, xe2x80x9cPolysar Butyl XL 10000xe2x80x9d which is of a highly crosslinked grade (having a gel content of 70% to 85% as measured using cyclohexane) is preferable.
An extender component used in the present invention is a softening agent for lowering the hardness of a rubber and a plasticizer usually added to a rubber composition.
Softening agents such as process oils (for example, of paraffin, naphthene and aromatic series) and vegetable oils (for example, castor, rapeseed, soybean, palm, coconut, peanut, cottonseed, pine and olive oils and Japan wax), as well as synthetic softening agents may be used in the present invention.
Plasticizers such as derivatives of phthalic, adipic, azelaic, sebacic, maleic, fumaric, trimellitic, citric, itaconic, oleic, ricinoleic, stearic, phosphoric, glutaric and glycolic acids as well as glycerin and epoxy derivatives may be used in the present invention.
Among these extender components, those having a good compatibility with the partially crosslinked butyl rubber, i.e. those having a small solubility parameter are preferable. It is also possible to use a mixture of two or more of the above components for the present invention.
An extender component is preferably a softening agent, more preferably a process oil, and most preferably process oils of a paraffin series.
The extender component is used in an amount of 50 to 200 parts by weight and preferably 70 to 150 parts by weight, per 100 parts by weight of a partially crosslinked butyl rubber. A desired low hardness is not accomplished if the amount of the extender component is less than 50 parts by weight. On the other hand, a kneading property is deteriorated and the extender component tends to easily bleed if the amount of the extender component is greater than 200 parts by weight.
A rubber composition according to the present invention may further contain fillers, vulcanizing agents, processability-improving polymers and/or other additives for rubber in addition to the above components.
Fillers are used for reinforcing a rubber material and include inorganic fillers such as carbon black, clay, talc, calcium carbonate and silica, organic fillers such as powdered cork, cellulose and ebonite, and other fillers, and reinforcing agents usually added to a rubber composition. These fillers may be used in an amount of 10 to 100 parts by weight and preferably 10 to 50 parts by weight, per 100 parts by weight of a partially crosslinked butyl rubber.
Vulcanizing agents include organic peroxides as well as vulcanizing systems usually used for vulcanization of a butyl rubber such as sulfur, quinoids, resins, sulfur donors and low-sulfur high-performance vulcanization accelerators. However, sulfur is not desirable because it corrodes other parts of instruments. On the other hand, organic peroxides provide a rubber material having a small compression set, and therefore, they are preferably used in the present invention. If the peroxides are used for crosslinking and curing conventional butyl rubbers, the main chains of the rubbers degrade and satisfactorily cured products are not obtained. Accordingly, a sulfur type vulcanizing agent was hitherto used for the vulcanization. Since, however, the present rubber composition contains a partially crosslinked butyl rubber as a main component, it is possible to use peroxides as vulcanizing agents.
Organic peroxides include, for example, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane and dicumyl peroxide. These organic peroxides may be used in an amount of 2 to 10 parts by weight and preferably 2 to 5 parts by weight, per 100 parts by weight of a partially crosslinked butyl rubber.
Processability-improving polymers are, in particular, those for improving the kneading property of a rubber composition. Such polymers include natural rubbers, synthetic rubbers (for example, IR, BR, SBR, CR, NBR, IIR, EPM, EPDM, acrylic rubber, EVA, urethane rubber, silicone rubber, and fluororubber) and thermoplastic elastomers (for example, of styrene, olefin, vinyl chloride, ester, amide, and urethane series). These processability-improving polymers may be used in an amount of 0 to 100 parts by weight, preferably 0 to 50 parts by weight and most preferably 0 to 30 parts by weight, per 100 parts by weight of a partially crosslinked butyl rubber.
Other additives for rubber include conventional additives for rubber such as anti-aging agents, anti-oxidants, lubricants and flame retarders. These additives may be used in an amount usually used.
The rubber composition of the present invention can be prepared by kneading the above components in any known manner. For example, it can be prepared using an open roll or an enclosed kneader (for example, internal mixer, kneader or Banbury mixer) by firstly masticating a partially crosslinked butyl rubber, then adding a polymer for improving the kneading property to the butyl rubber, further adding fillers, extender components, vulcanizing agents and other additives for rubber to the mixture, and kneading the resulting mixture. Kneading temperatures may be in the range of 25xc2x0 C. to 120xc2x0 C.
A rubber vibration insulator can be prepared from the rubber composition, for example, by compression press molding and preferably by transfer molding. Molding temperatures may be in the range of 130xc2x0 C. to 170xc2x0 C., and molding periods may be within 20 minutes and preferably in the range of 4 to 10 minutes.